1. Field of the Invention
The present invention relates to an optical head apparatus that performs recording or reproduction of optical information recording media.
2. Description of the Prior Art
Generally, a rewritable type optical disc must monitor the quantity of incident light to the recording surface of the disc to secure the signal recording quality with high accuracy. For this reason, the accuracy of a system that monitors the light quantity using light output from the posterior end face of a laser chip used in a reproduction-only optical head is not high, and therefore it is necessary to monitor the light quantity using light radiated from the anterior end face of the laser chip (hereinafter referred to as xe2x80x9canterior lightxe2x80x9d).
On the other hand, while optical discs are attracting attention as large-capacity information memories, optical head apparatuses need to attend a demand for high-speed recording or reproduction of optical discs. To meet this demand, it is necessary to increase the speed of modulation of a semiconductor laser light source and at the same time improve responsivity of the above described monitoring of the anterior light.
A conventional optical pick up will be explained with reference to the attached drawings below. FIG. 14 shows an example of an outlined configuration of a conventional optical head, apparatus. A diverging beam 802 radiated from a semiconductor laser light source 801 passes through a parallel flat plate 803 placed diagonally to the optical axis and is converted to a parallel beam 805 by a collimate lens 804.
This collimated beam 805 is partially reflected by a polarized beam splitter 806 and enters into a photodetector 809. A beam 810, the major portion of the collimated beam 805, passes through the polarized beam splitter 806 and is converted to a circularly polarized beam by a xc2xc wavelength plate 811, and then condensed into an optical disc 814 through an objective lens 813 mounted on an actuator 812.
The beam reflected by the optical disc 814 passes through the objective lens 813 and is converted by the xc2xc wavelength plate 811 to a linearly polarized beam, which is orthogonal to the polarization plane of the outgoing radiation beam of the semiconductor laser light source 801 and entered into the polarized beam splitter 806.
Since the polarization plane of the incident beam entered into the polarized beam splitter 806 is orthogonal to the first half of the optical path, the incident beam is reflected by the polarized beam splitter 806, diffracted by a hologram element 815, branched into a positive 1st-order diffracted light 817 and negative 1st-order diffracted light 818 with the optical axis of the incident light as an axis of symmetry, then condensed by a detection lens 817, entered into signal detectors 820 and 821, respectively, to detect control signals such as focusing and tacking, and RF signals.
On the other hand, photodetector 809 that detects light reflected by the polarized beam splitter 806 acts as an output light quantity monitor of the semiconductor laser light source 801.
Here, the reason why the parallel plate 803 is placed diagonally to the optical axis of the incident beam between the semiconductor laser light source 801 and collimate lens 804 will be explained. Generally, as for a semiconductor laser used for a light source of the optical head apparatus, from the standpoint of an optical characteristic, mode west of an oscillated beam of a semiconductor laser element 901 differs between the semiconductor composition plane (X-Z axial plane) and the plane normal thereto (Y-Z axial plane) as shown in FIG. 15.
That is, while the mode west is a point that matches a specular surface 902 within the perpendicular (Y-Z axial plane), it is a point inside an activated layer 903 of the semiconductor laser element 901, that is, a point at a certain depth from the specular surface 902 into the resonator within the composition plane (X-Z axial plane).
Therefore, the converging point of the oscillated beam differs between the composition plane (X-Z axial plane) and the plane normal thereto (Y-Z axial plane), and thus an xe2x80x9castigmatic differencexe2x80x9d 904 in optical terms is produced.
When an astigmatic difference occurs, the beam spot is distorted into a flat, vertically or horizontally oblong spot. Therefore, the beam spot spans mutually neighboring recording tracks of an optical disc, causing a problem of deteriorating a signal characteristic.
It is for this reason that in FIG. 14, the parallel plate 803 is placed inclined at a predetermined angle in the reverse direction in order to correct the astigmatism of the light beam radiated from the semiconductor laser 801.
Moreover, another method proposed to correct such astigmatism of a light beam is canceling out the astigmatism of the light spot by inserting a cylindrical lens in the same optical path of the laser beam.
The above described conventional optical head apparatus has the following problems:
Generally, when recording a signal on a rewritable type optical disc, it is necessary to secure sufficient optical power on the disc, and therefore the light utilization efficiency of the optical head must be secured.
However, the configuration of the above described conventional example performs no beam shaping, and therefore abandons a portion of light in the outer regions for reasons related to the design of the objective lens, which means a loss of light quantity.
Furthermore, a part of the beam within the effective aperture is reflected and used by the photodetector 809 to monitor the light quantity, which increases the loss all the more. To avoid this, lowering the light quantity to be conducted to the light quantity monitor and increasing the light quantity within the effective aperture will deteriorate the S/N ratio of the monitor signal.
Moreover, increasing the speed of laser modulation requires the responsivity of the anterior light monitor itself to be improved. For this reason, it is preferable to reduce the photoreception area of the photodetector and input a condensed beam in order to improve the response frequency characteristic of optical detection.
However, exposing the photodetector to an excessively condensed beam will increase the light intensity per unit area of the detector surface, increasing the carrier density on the photoreception surface of the detector, which then becomes saturated causing the traveling speed of carriers to slow down. That is, condensing the beam on the detector excessively may cause a problem of deteriorating the response frequency characteristic of optical detection.
Furthermore, all the above described methods to correct the astigmatism of a light beam produced by an astigmatic difference among the semiconductor laser elements above must provide special parts such as a transparent parallel plate and cylindrical lens separately, causing an additional problem of unavoidably increasing the number of parts, hence cost increase.
In addition, since the photodetector for an RF signal, focusing or tracking control signals is provided apart from the photodetector for laser light quantity monitoring, which increases the number of parts and complicates the optical system, making it difficult to reduce the size of the optical head.
The present invention has been implemented taking into account these problems of the conventional optical head apparatus and it is an object of the present invention to provide an optical head apparatus with high light utilization efficiency.
It is another object of the present invention to provide a compact optical head apparatus.
It is still another object of the present invention to provide an optical head apparatus with an excellent response frequency characteristic of optical detection.
Therefore one aspect of the present invention is an optical head apparatus, comprising:
a semiconductor laser light source;
a photodetector that receives at least one part of light from said semiconductor laser light source;
a light reflection element provided with a peripheral section that reflects peripheral light of to the light from said semiconductor laser light source and condenses it into said photodetector and a central section that transmits central light of the light from said semiconductor laser light source; and
a condenser lens that condenses the light that passes through said light reflection element onto an optical disc,
wherein:
each surface of the central section of said light reflection element has a flat shape; and
at least one surface of the peripheral section of said light reflection element has a spherical or non-spherical shape.
Therefore another aspect of the present invention is an optical head apparatus, comprising:
a semiconductor laser light source;
a photodetector that receives at least one part of light from said semiconductor laser light source;
a light reflection element provided with a function of reflecting peripheral light of the light from said semiconductor laser light source and condensing it into said photodetector and a function of transmitting the central light of the light from said semiconductor laser light source; and
a condenser lens that condenses the light that passes through said light reflection element onto an optical disc,
characterized in that said semiconductor laser light source and said photodetector are formed in one package.
Therefore still another aspect of the present invention is an optical head apparatus, comprising:
a semiconductor laser light source;
a plurality of photodetectors placed adjacent to said semiconductor laser light source;
a reflection type hologram element provided with a peripheral section that reflects and diffracts peripheral light of the light from said semiconductor laser light source and condenses it into one of said plurality of photodetectors and a central section that transmits central light of the light from said semiconductor laser light source; and
a condenser lens that condenses the light that passes through the central section of said reflection type hologram element onto an optical disc,
wherein:
said photodetector that receives said reflected and diffracted light is placed closer, with respect to said semiconductor laser light source, in the direction of the major axis of an ellipse than in the direction of the minor axis of the ellipse of an elliptic far field pattern of outgoing light from said semiconductor laser light source; and
the photodetector that receives signal light from said optical disc is placed closer, with respect to said semiconductor laser light source, in the direction of the minor axis of the ellipse than in the direction of the major axis of the ellipse of an elliptic far field pattern of outgoing light from said semiconductor laser light source.